Add a new feature flag for the expand button in support lib.

- check the feature flag to determine whether to use the existing
progressive disclosure mixin or use the expand button in the support
library.
- add corresponding preference xml files that add the new preference
group attribute to use with the expand button in the support lib.

Bug: 63985174
Test: make RunSettingsRoboTests
Change-Id: Ida6eb6182a8066ad1413b7f6142512345fd914d0

tests/robotests/src/com/android/settings/notification/SoundSettingsTest.java

@@ -21,6 +21,7 @@ import android.content.Context;
 import android.media.AudioManager;
 import android.os.UserManager;
 import com.android.settings.R;
+import com.android.settings.dashboard.ProgressiveDisclosureMixin;
 import com.android.settings.testutils.SettingsRobolectricTestRunner;
 import com.android.settings.TestConfig;
 import com.android.settings.testutils.XmlTestUtils;
@@ -30,6 +31,7 @@ import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.robolectric.RuntimeEnvironment;
 import org.robolectric.annotation.Config;
+import org.robolectric.util.ReflectionHelpers;
 
 import java.util.List;
 
@@ -59,7 +61,10 @@ public class SoundSettingsTest {
 
         final List<String> niks = SoundSettings.SEARCH_INDEX_DATA_PROVIDER
                 .getNonIndexableKeys(context);
-        final int xmlId = (new SoundSettings()).getPreferenceScreenResId();
+        SoundSettings settings = new SoundSettings();
+        ReflectionHelpers.setField(settings, "mProgressiveDisclosureMixin",
+                mock(ProgressiveDisclosureMixin.class));
+        final int xmlId = settings.getPreferenceScreenResId();
         final List<String> keys = XmlTestUtils.getKeysFromPreferenceXml(context, xmlId);
         keys.addAll(XmlTestUtils.getKeysFromPreferenceXml(context,
                 R.xml.zen_mode_settings));